The present invention relates to combustion control systems and more specifically to combustion control systems for gas burners, in particular for cooker hobs.
GB2249382 A describes a gas burner ignition system with a safety device. This safety device includes a safety valve that keeps the flow of gas to the burner open when the thermocouple at this burner detects that there is a flame. The axial movement of the tap opening control is needed to open the safety valve. When this valve is open, a voltage that keeps the valve open while the thermocouple warms up is applied to it by means of a pulse generator. Then, it is the actual thermocouple that supplies the safety valve directly with the voltage needed for it to be kept open.
JP09112912 A shows a circuit that monitors the voltage of the thermistor located at the cooker burner and closes the solenoid valve for the flow of gas when the tap opening detector switch is at ON and the value of the thermistor voltage is below a given level.
An initial object of the invention is to provide a control circuit for gas burners, in particular for cooker hobs, with a fail-safe system, as defined in the claims. Said control circuit includes a microcontroller that activates the burner spark generator circuit and acts on the gas flow valves associated with the burners in accordance with the signals it receives from the flame detector circuits of these burners and from the tap opening detector switches associated with each burner. In addition, the microcontroller also monitors the flow of current at least at one of the points of the control circuit, so as to close the respective valve in the event of detecting any anomaly.
The circuit of the invention is specially designed for use with low voltage DC valves, especially if said valves are adapted to operate referenced to ground.
In general, flame detector circuits include a thermocouple. In the circuit of the invention said thermocouples do not act directly on the valves, but rather it will be the microcontroller that acts on them in accordance with, amongst other signals, the signal obtained from said thermocouples.
The gas flow tap associated with each burner consists of a rotary control operated by the user and of a valve that is actuated directly by the control circuit. An axial movement does not have to be applied as well to the rotary control as on traditional systems. On being turned, the status of said switch being read by the microcontroller of the control circuit.
For flow to take place to a burner, two conditions have to be met: the user should open the tap corresponding to said burner and the electronic circuit should open the valve of said burner. This makes it possible to use a single valve for all the burners.
An operating voltage is applied to the valves to open them and, once they are open, a maintenance voltage is applied to them. Said maintenance voltage is lower than the operating voltage but sufficient to keep the valves open. In this way, a more economic and more compact design is obtained (the volume of the power supply is reduced). The valve described in the Spanish Patent Application with Application No. P9900547 is an example of a valve that may be acted upon using one voltage to open it and another to keep it open. On said valve, the energy required to attract its armature is greater than the energy needed to keep it attracted.
For each valve, the control circuit of the invention includes a switch which, when closed, supplies the operating voltage to said valves, a switch which, when closed, supplies the maintenance voltage to said valves, and a switch which, when closed, short-circuits said valves, so that they are de-energized.
In this way, both the operating voltage and the maintenance voltage of each valve are supplied by means of the closing of a switch and the opening of a second switch. This means that the system is safe against the failure of any of said switches. Thus, even if one of the switches fails, there is another switch on which the microcontroller will act in the event of such a failure, so that no voltage is supplied to the valve and all possibility of gas leakage is thereby prevented. Furthermore, in the event of the failure of a switch, the system is safe against the failure (short circuit, open circuit, etc.) of any components of the control circuit other than these.
In the preferred embodiment of the invention, one terminal of the valves is connected to the chassis where the burners are generally located, whereby the return of the operating and maintenance currents is carried out by way of said chassis. Said chassis is usually connected to earth.
In general, one terminal of the flame sensors (usually thermocouples) and one terminal of the tap switches are also connected to said chassis. Connecting one terminal of the valves, the flame sensors and the tap switches to the chassis offers advantages in terms of economy and reliability, since:
As the insulation of the valves and the flame sensors in respect of the supply system is provided by means of the supplier transformer, the construction of said valves and said flame sensors is simplified.
The signals from the valves, flame sensors and tap switches are being returned through the chassis, so the number of wires and connecting components is cut by half.
So that failures in the control circuit may be detected, the microcontroller checks the current flow at least at one point of said control circuit. In view of the arrangement in the circuit of the operating switch, the maintenance switch and the switch that short-circuits each valve, by checking the current flow through each valve the microcontroller can determine the state of the three switches corresponding to said valve.
To ensure that said valves are closed also in case it is the actual microcontroller that fails, at least one circuit is added that acts on one or more switches depending on whether said circuit receives pulses or not from the microcontroller, the closing of the respective valves being brought about when said circuits cease to receive pulses. The switches that short out the valves have a circuit of these characteristics, whereby the valves remain short-circuited in the event of failure of the microcontroller.
The use of a microcontroller successfully endows the control circuit with a flexibility that enables functions to be added to said control circuit. Thus, for instance, the following additional features will be added: reignition attempt during a given period of time if the flame accidentally goes out, indication of the state of the burners and the cooker hob by display, keys for locking and releasing the hob, etc.
A second object of the invention is also to provide a dual fail-safe control circuit, as defined in the claims. Thus, providing the circuit with a few additional items succeeds in making the circuit of the invention safe against any failure of the system components, but also against two failures of these components, even if they occur simultaneously.